feat(portfolio): C2 单股上限约束真实执行（Portfolio v1.1）

- portfolio.allocate_with_max_position：无上限=等权（与原实现一致）；有上限=迭代
  把超过 cap×当日组合市值的标的封顶并把剩余现金在其余标的中再分配，超出留现金
- TopKBacktestRunner 买入按约束分流（默认等权路径位级不变，回归数值保持）
- unimplemented 只保留行业上限（依赖行业元数据注入，如实标注）
- tests/test_portfolio_constraints.py（分配数值/封顶留现金/回测持仓权重≤上限+容差、
  config_snapshot）+ 旧断言更新（单股不再标注）；全量 pytest 通过

backend/app/quant/local_engine.py

@@ -34,7 +34,11 @@ from app.quant.composite import (  # noqa: F401 —— re-export（模块化后
     cross_sectional_zscore,
 )
 from app.quant.evaluation import run_factor_test
-from app.quant.portfolio import equal_weight_budget, unimplemented_notes
+from app.quant.portfolio import (
+    allocate_with_max_position,
+    equal_weight_budget,
+    unimplemented_notes,
+)
 
 TRADING_DAYS = 252
 _DEFAULT_UNIMPLEMENTED = [
@@ -204,8 +208,25 @@ class TopKBacktestRunner:
 
         # BUY 信号/成交记录：意图入选（filled）或意图被拒（原因）；替补成交同样如实记录
         if targets:
-            budget = equal_weight_budget(cash, len(targets))
+            cap = self.spec.portfolio.max_position_pct
+            if cap is None:
+                # 默认等权（与原实现位级一致，保持回归数值不变）
+                budget = equal_weight_budget(cash, len(targets))
+                spends = {s: budget for s in targets}
+                total_spend = budget * len(targets)
+            else:
+                # Portfolio v1.1：按单股上限（相对当日组合市值）分配，超出部分留现金
+                equity_now = cash + sum(
+                    float(self.close.at[d, s] * qty)
+                    for s, qty in shares.items()
+                    if qty > 0 and not _nan(self.close.at[d, s])
+                )
+                spends = allocate_with_max_position(cash, targets, equity_now, cap)
+                total_spend = sum(spends.values())
             for s in targets:
+                budget = spends[s]
+                if budget <= 1e-9:
+                    continue
                 c = float(close_d[s])
                 price_in = c * (1 + self.costs.slippage_rate)
                 invest = budget * (1 - self.costs.commission_rate)
@@ -217,7 +238,7 @@ class TopKBacktestRunner:
                     ActionRecord(date=day, symbol=s, signal="BUY", filled=True,
                                  price=round(price_in, 4))
                 )
-            cash -= budget * len(targets)
+            cash -= total_spend
         for sym in picks:
             if sym in target_set:
                 continue